Bar Screen Design 4qqt

MANUAL BAR SCREEN DESIGN

Objective: Screens are used in wastewater treatment for the removal of coarse solids. General design basics: S.N o

Description Bar spacing is in range The screen is mounted at an angle of from horizontal Bar size Minimum approach velocity in the bar screen channel to prevent grit deposition Maximum velocity between the bars to prevent washout of solids through the bars

Standard value 2 to 5 45-70

Unit

1 thick x 2.5 wide 0.45

cm

0.9

m/s

cm degree

m/s

Example: Average Flow = 25000 m3/day Peak flow factor= 1.45 Peak flow = 36250 m3/day Velocity considered= 0.6 m/s Angle of inclination is 50o Depth/width ratio = 1.5 Space between bars= 30 mm and thickness of bar= 10 mm 1. The cross section of the bar screen channel is determined from the continuity equation Q = Ax V Where Q= flow rate, m3/sec A= Channel cross section area, m2 V= Velocity, m/s

A=Q/V A= 0.419 m3/s 0.6 m/s A=0.699 m2

Area= channel width, m x channel depth, m d/W=1.5 d= 1.5 W Area = width x 1.5 width Width2 = 0.699 m2 1.5 W= 0.682 m d= 0.682 m x 1.5 d=1.023 m

2. The cross section of the bar screen is given by the following equation Ab = A/Sinθ Where Ab = bar screen cross section, m2 Ө = inclination angle of the screen Ab = 0.699 m2/ sin 50 Ab = 0.912 m2

3. The net area of the bar screen available for flow is given by the following equation: Anet = Ab

S S+tbar

Where

tbar = thickness of the screen bars, m S = space between bars, m

Anet = 0.912 m2

30 mm 30mm+ 10mm

Anet

= 0.684 m2

From continuity equation: V A= Vb Anet Vb = 0.6 * 0.699/0.684 Vb =0.613 m2 <0.9 it’s OK 4. The head loss through the bar screen is given by the following equation:

H1 =

Vb2- V2 2g

1 0.7

Where H1= Head loss V= approach velocity, m/s Vb= Velocity through opening, m/s

g= acceleration due to gravity, m/s2 For clear screen: H1 =

0.6132- 0.62

1

2*9.81

0.7

H1=0.001085 m

For 45% clogged screen:

Anet = 55% Anet V A= Vb Anet Vb = 0.6 * 0.699/0.684*0.55 Vb= 1.11 m/s H1 =

1.112- 0.62 2*9.81

1 0.7

H1=0.0631 m 5. The number of bars in the screen is given by the following equation: n tbar + (n-1)S = W n*10+(n-1)30=682 n 10+ n 30=682+30 n=17.8 bars=say 18 nos.

6. Wetted perimeter P= w + 2 d P= wetted perimeter,m W= width of channel, m d= depth of channel, m

P=0.682+2*1.023 P= 2.728 7. Hydraulic radius R= wd/w+2d R=0.683*1.023/2.725 R=0.256 m 8. Slope Manning equation v = kn / n R2/3 S1/2

(1)

where v = cross-sectional average velocity (m/s) kn = 1.0 for SI units A = cross sectional area of flow (m2) n = Manning coefficient of roughness R = hydraulic radius (m) S = slope of pipe (m/m) N= 0.013 S1/2= v* n/ R2/3 S1/2= 0.6*0.013/0.2560.66 S1/2= 0.0078/0.4068 S= 0.01917 m/m